With a budget of $500
Refurbished Dell
i7 Intel processor
8 gigs of at least DDR3 ram
250 gig SSD with a SATA 500 gig drive (best case would be a new Samsung 1 TB SSD and whatever the computer came with)
some sort of interface to plug a guitar or mic into.

I looked on Amazon...

From Amazon for $429. This looks likes a pretty solid machine. Needs a bigger drive. Samsung SSD 860 EVO 1TB 2.5 Inch SATA III Internal SSD (MZ-76E1T0B/AM) on sale, black Monday $99.

Dell Optiplex Gaming PC 3020 SFF Computer - Core i7 4770 3.4 GHz, NVIDIA GT 1030 2GB, 8GB RAM, 256GB SSD, HDMI, DVI, VGA, DVD-ROM Drive, Keyboard, Mouse, WiFi, Windows 10 Professional(Renewed)

This will run most anything and there are ways to deal with the limited amount of ram.

Cheers,

Billy